Method for liquefying a hydrocarbon-rich stream

ABSTRACT

A method for liquefying a hydrocarbon-rich stream, in particular a natural gas stream, is disclosed. A liquefaction of the hydrocarbon-rich stream takes place countercurrent to a refrigerant mixture cycle cascade consisting of two or three refrigerant mixture cycles. No additional process steps are involved in the heat exchange between the hydrocarbon-rich stream which is to be precooled and the refrigerant mixture of the first refrigerant mixture cycle.

This application claims the priority of International Application No.PCT/EP2005/013313, filed Dec. 12, 2005, and German Patent Document No.10 2005 000 647.7, filed Jan. 3, 2005, the disclosures of which areexpressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for liquefying a hydrocarbon-richstream, in particular a stream of natural gas, where the liquefaction ofthe hydrocarbon-rich stream takes place countercurrent to a refrigerantmixture cascade consisting of two refrigerant mixture cycles and wherethe first refrigerant mixture cycle is used for precooling and thesecond refrigerant mixture cycle is used for liquefying and supercoolingthe hydrocarbon-rich stream to be liquefied.

The invention further relates to a method for liquefying ahydrocarbon-rich stream, in particular a stream of natural gas, wherethe liquefaction of the hydrocarbon-rich stream takes placecountercurrent to a refrigerant mixture cycle cascade consisting ofthree refrigerant mixture cycles and where the first of the threerefrigerant mixture cycles is used for precooling, the secondrefrigerant mixture cycle is used for the actual liquefaction and thethird refrigerant mixture cycle for supercooling the liquefiedhydrocarbon-rich stream.

In what follows, the term “precooling” should be understood to mean thecooling of the hydrocarbon-rich stream down to a temperature at whichthe separation of heavy, or higher-boiling, hydrocarbons takes place.The subsequent further cooling of the hydrocarbon-rich stream to beliquefied hereinafter comes under the term “liquefaction”.

Generic natural gas liquefaction methods in which the liquefaction ofthe hydrocarbon-rich stream takes place countercurrent to a refrigerantmixture cycle cascade consisting of two refrigerant mixturecycles—generally designated as dual-flow LNG process—are sufficientlywell known to the person skilled in the art; U.S. Pat. No. 6,105,389 canbe named as an example.

The same applies to generic natural gas liquefaction methods in whichthe liquefaction of the hydrocarbon-rich stream takes placecountercurrent to a refrigerant mixture cycle cascade consisting ofthree refrigerant mixture cycles; the natural gas liquefaction methoddescribed in German disclosure 197 16 415 can be named as an examplethereof.

With the citation of the two aforementioned patents, the content oftheir disclosure is hereby expressly incorporated by reference hereininto the disclosure of this patent application.

Normally—as explained for example in the aforementioned U.S. Pat. No.6,105,389—the exchange of heat in precooling, liquefaction andsupercooling takes place in a combined multi-stream heat exchanger. Allprocess streams are taken through a common type of heat exchanger.According to the required, or desired, capacity, several identical heatexchanger (units) located in parallel are provided.

As a matter of principle it holds true that natural gas liquefactionplants in which plate heat exchangers are used for precooling requireseveral heat exchangers arranged in parallel above about 0.2 mtpa LNGcapacity since the heating surface per heat exchanger block is limited.Consequently, for a liquefaction capacity of 5 mtpa 20 to 30 heatexchanger blocks arranged in parallel must be provided, as long as eachcontains all the process streams.

However, the following disadvantages result:

-   -   uniform distribution;    -   high cost of pipelines;    -   operating stability at part load operation, in particular the        problem of entrainment; and    -   thermal imbalance when operating the plant only with precooling,        i.e., without liquefaction and cooling because of missing        process streams; this is of importance in particular during the        start-up procedure for the liquefaction process.

The object of the present invention is to specify generic methods forliquefying a hydrocarbon-rich stream in which the aforementionedproblems can be avoided.

To achieve this object, it is provided in the case of genericliquefaction processes in which liquefaction of the hydrocarbon-richstream takes place countercurrent to a refrigerant mixture cycle cascadeconsisting of two or three refrigerant mixture cycles that no additionalprocess streams be involved in the heat exchange between thehydrocarbon-rich stream to be precooled and the refrigerant mixture ofthe first refrigerant mixture cycle.

By means of the method in accordance with the invention which representsa “specialization” of the heat exchangers, the number of heatexchangers, or heat exchanger blocks, for the same process task can bereduced substantially. As a concomitant result, the required expenditurefor pipelines can be reduced.

The number of blocks per heat exchanger type can be kept below 16 at aliquefaction capacity up to 10 mtpa, preferably between 2 and 8 blocks.This allows symmetrical pipelines at an appropriate expenditure.

BRIEF DESCRIPTION OF THE DRAWINGS

The method in accordance with the invention and additional embodimentsof the same are explained in more detail hereinafter using theembodiments shown in FIGS. 1 to 5.

FIG. 1 shows a natural gas liquefaction method in which the liquefactiontakes place countercurrent to a refrigerant mixture cycle consisting oftwo refrigerant mixture cycles;

FIG. 2 shows a natural gas liquefaction method in which the liquefactiontakes place countercurrent to a refrigerant mixture cycle consisting ofthree refrigerant mixture cycles;

FIG. 3 shows a natural gas liquefaction method as explained using FIG. 2and in which at least one refrigerant mixture partial stream from thesecond refrigerant mixture cycle is used for the precooling of thenatural gas; and

FIGS. 4/5 show natural gas liquefaction methods as explained using FIG.2 in which the cooling of the refrigerant mixture of the second andternary refrigerant mixture cycles takes place in dual-streamexchangers.

DETAILED DESCRIPTION OF THE DRAWINGS

In the embodiment of the method in accordance with the invention shownin FIG. 1, the natural gas stream to be cooled and liquefied is takenover line 1 to a first heat exchanger E1. Here the natural gas stream iscooled countercurrent to a partial stream P3 of the refrigerant mixtureof the first refrigerant mixture cycle. Subsequently the natural gasstream is taken over line 2 to a second heat exchanger E2 in which it iscooled in succession countercurrent to two partial streams P5 and P7 ofthe refrigerant mixture of the first refrigerant mixture cycle.

The natural gas stream cooled in this way is subsequently taken overline 3 to a further heat exchanger E5 and liquefied in the exchangercountercurrent to the refrigerant mixture L2 of the second refrigerantmixture cycle and supercooled as necessary. Subsequently to this theliquefied natural gas stream (LNG) is taken over line 4 to its furtheruse and/or to storage.

The last described liquefaction and supercooling of the precoolednatural gas stream takes place in the case of the embodiment shown inFIG. 1 countercurrent to the second refrigerant mixture cycle L1 to L4of the refrigerant mixture circuit cascade, where the refrigerantmixture compressed by means of single- or multi-stage compression LV isfirst taken to an aftercooler LK and subsequently over line 4 to a heatexchanger E3. A cooling and liquefaction of the refrigerant mixture ofthe second refrigerant mixture cycle takes place in the exchangercountercurrent to partial streams P9, P11 and P13 of the refrigerantmixture of the first refrigerant mixture cycle, which are available atsuitable temperature levels.

The thus cooled and liquefied refrigerant mixture of the secondrefrigerant mixture cycle is subsequently taken over line L1 to heatexchanger E5 already mentioned, supercooled countercurrent to itself,drawn off over line L2 from heat exchanger E5, expanded and again takenthrough heat exchanger E5 countercurrent to the natural gas stream to beliquefied and if necessary supercooled. Subsequently the refrigerantmixture is drawn off over line L3 and taken to the single- ormulti-stage circuit compressor LV already mentioned.

The partial streams P3, P5 and P7 already mentioned of the refrigerantmixture of the first refrigeration cycle which serve to precool thenatural gas stream 1 or 2 in heat exchangers E1 and E2 are reunited inthe multi-stage compressor unit PV of the first refrigerant mixturecycle.

The refrigerant mixture stream compressed in compression PV is takenover line P1 to a condenser PK and subsequently over line P2 to thefirst of three heat exchangers E4A, E4B and E4C. After each of the threeaforementioned heat exchangers, partial streams of the refrigerantmixture at suitable temperature levels are drawn off through the linesP3, P5 or P7, expanded and subsequently—as already described—takenthrough heat exchangers E1 and E2 for the purpose of precooling thenatural gas stream 1 or 2 which is to be liquefied.

To supercool the aforementioned refrigerant mixture partial streams P3,P5 and P7, partial streams P15, P17 and P19 are drawn off from them inturn, expanded and taken countercurrent through the three aforementionedheat exchangers E4A, E4B or E4C. These partial streams are subsequentlyin turn admixed to the particular streams from which they were drawn offover lines P16, P18 and P20 before compression PV.

Further developing the methods in accordance with the invention, it isprovided that no further process streams be involved in the heatexchange E4A, E4B and E4C of the refrigerant mixture P2 of the firstrefrigerant mixture cycle to be cooled countercurrent to itself.

For the purpose of cooling, or supercooling, the refrigerant mixture L4of the second refrigerant mixture cycle in heat exchanger E3, partialstreams are similarly drawn off from the three refrigerant mixturepartial streams P3, P5 and P7 over lines P9, P11 and P13, expanded andtaken through the heat exchanger E3 countercurrent to the refrigerantmixture L4 of the second refrigerant mixture cycle. These refrigerantmixture partial streams are also subsequently admixed over lines P10,P12 and P14 to the refrigerant mixture partial streams in the lines P4,P6 and P8 before compression PV.

The embodiment of the method in accordance with the invention shown inFIG. 2 differs from the one shown in FIG. 1 in that an additionalrefrigerant mixture stream is now provided for the supercooling of theliquefied natural gas stream. Consequently, in what follows only thedifferences between the embodiments shown in FIGS. 1 and 2 will bediscussed.

In the case of the example of the method shown in FIG. 2, theliquefaction of the natural gas stream precooled in heat exchangers E1and E2 takes place in heat exchanger E5 countercurrent to therefrigerant mixture stream of the second refrigerant mixture cycle.Subsequently the liquefied natural gas stream is taken over line 4 to afurther heat exchanger E6, supercooled in the heat exchangercountercurrent to the refrigerant mixture stream S3 of the thirdrefrigerant mixture cycle and subsequently taken over line 5 to itsfurther use and/or storage.

As already explained using the first and the second refrigerant mixturecycle, the refrigerant mixture of the third refrigerant mixture cycle isalso initially compressed in a single- or multi-stage compression SV andtaken to an aftercooler SK and subsequently to heat exchanger E3 overline S1. The refrigerant mixture—jointly with the refrigerant mixturefrom the second refrigerant mixture cycle—is cooled in the heatexchanger countercurrent to several refrigerant mixture partial streamsof the first refrigerant mixture cycle and at least partially condensed.

The cooled refrigerant mixture of the third refrigerant mixture cycle istaken to heat exchanger E5 over line S2, cooled further here, completelycondensed and subsequently supercooled in heat exchanger E6. Thesupercooled refrigerant mixture is drawn off from the latter over lineS3, expanded and again taken through heater exchanger E6 countercurrentto the natural gas stream to be supercooled. Subsequently the heatedrefrigerant mixture of the third refrigerant mixture cycle is againtaken over line S4 to compression SV, which has already been described.

FIG. 3 shows an embodiment of the method in accordance with theinvention in which a partial stream of the refrigerant mixture of thesecond refrigerant mixture cycle—in addition to the refrigerant mixturefrom the first refrigerant mixture cycle—is used for precooling thenatural gas stream to be liquefied.

For this purpose, a refrigerant mixture partial stream from therefrigerant mixture stream cooled in heat exchanger E3 is drawn off overline L5, expanded and taken at a suitable temperature level through heatexchanger E2 in counterflow to the natural gas stream 2 which is to becooled. The heated refrigerant mixture partial stream is subsequentlytaken to compression LV over line L6.

A further partial stream of refrigerant mixture L1 of the secondrefrigerant mixture cycle cooled in heat exchanger E3 is drawn off overline L7, expanded and taken to heat exchanger E3 for the purpose ofpreparing refrigerant. This refrigerant mixture partial stream is takenover line L8 to the compressor unit LV after passing through heatexchanger E3.

FIGS. 4 and 5 show embodiments of the method in accordance with theinvention in which the cooling of refrigerant mixture L4 of the secondrefrigerant mixture cycle takes place countercurrent to refrigerantmixture partial streams from the first refrigerant mixture cycle Pa,Pa′, Pb or Pb′ and the cooling of refrigerant mixture S1 from the thirdrefrigerant mixture cycle takes place countercurrent to refrigerantmixture partial streams from the first refrigerant mixture cycle Pc,Pc′, Pd or Pd′ in dual-stream exchangers E3A, E3B, E3C or E3D. Thedual-stream exchangers E3A, E3B, E3C or E3D are preferably configured asplate exchangers.

This method of proceeding requires a redesign of heat exchangers E2 andE3; the remaining heat exchangers can in principle remain unchanged.

This embodiment of the method in accordance with the invention forliquefying a hydrocarbon-rich stream has the advantage that allrefrigerant mixture partial streams of the first refrigerant mixturecircuit Pa, Pb, Pc and Pd are carried in separate flow passages ofdual-stream exchangers E3A, E3B, E3C and E3D which are optimized for theparticular task and thereby substantially improve performance, inparticular at startup and at part load. Against this, there is thedisadvantage that the greater number of heat exchanger models causesincreased engineering expense.

The embodiment shown in accordance with the invention of the method inFIG. 5 for liquefying a hydrocarbon-rich stream differs from the oneshown in FIG. 4 only in that the refrigerant mixture of the secondrefrigerant mixture cycle is evaporated at two different temperaturelevels. As a result, the heat exchanger E5 shown in FIG. 4 is dividedinto two heat exchangers E5A and E5B.

The inventive methods for liquefying a hydrocarbon-rich stream, inparticular a natural gas stream, in contrast to the known collectivepipelines which in the event of different pressure drops cause faultydistribution because of the non-symmetrical pipelines, allow therealization of sufficiently symmetrical pipelines and thus appropriateequal distribution by avoiding different pressure drops.

With the monobloc concept only one refrigerant mixture stream isresponsible for several streams to be cooled per heat exchanger section.As a result, the heat output of the individual refrigerant mixturestreams is higher than is compatible with stable entrainment.Entrainment is economical only in a load range of 1:3. Faultyentrainment in refrigerant circuits leads to demixing of gas and liquidand can compromise the stability of the operation and even themechanical strength of a heat exchanger.

In addition, the inventive methods reduce complexity with respect to thenecessary heat exchangers since predominantly only two-stream exchangersare used; as a result a thermal imbalance can largely be avoided in theevent of the failure of individual circuits.

1-10. (canceled)
 11. A method for liquefying a hydrocarbon-rich stream,in particular a natural gas stream, wherein a liquefaction of thehydrocarbon-rich stream takes place countercurrent to a refrigerantmixture cycle cascade consisting of two refrigerant mixture cycles andwherein a first refrigerant mixture cycle is used for precooling and asecond refrigerant mixture cycle is used for liquefaction andsupercooling of the hydrocarbon-rich stream to be liquefied, wherein noadditional process steps are involved in a heat exchange between thehydrocarbon-rich stream which is to be precooled and the refrigerantmixture of the first refrigerant mixture cycle.
 12. A method forliquefying a hydrocarbon-rich stream, in particular a natural gasstream, wherein a liquefaction of the hydrocarbon-rich stream takesplace countercurrent to a refrigerant mixture cycle cascade consistingof three refrigerant mixture cycles and wherein a first of the threerefrigerant mixture cycles is used for precooling, a second refrigerantmixture cycle for an actual liquefaction and a third refrigerant mixturecycle for supercooling the liquefied hydrocarbon-rich stream, wherein noadditional process steps are involved in a heat exchange between thehydrocarbon-rich stream which is to be precooled and the refrigerantmixture of the first refrigerant mixture cycle.
 13. The method accordingto claim 11, wherein at least one partial stream of the refrigerantmixture of the second refrigerant mixture cycle is used for theprecooling of the hydrocarbon-rich stream, wherein the partial streamfrom the refrigerant mixture of the second refrigerant mixture cycle isinvolved in the heat exchange between the hydrocarbon-rich stream andthe refrigerant mixture of the first refrigerant mixture cycle.
 14. Themethod according to claim 11, wherein the heat exchange between thehydrocarbon-rich stream to be precooled and the refrigerant mixture ofthe first refrigerant mixture cycle is implemented in at least onestraight-tube heat exchanger.
 15. The method according to claim 14,wherein the heat exchanger is a plate heat exchanger or a coil-type heatexchanger.
 16. The method according to claim 11, wherein no additionalprocess streams are involved in a heat exchange countercurrent to itselfof the refrigerant mixture of the first refrigerant mixture cycle. 17.The method according to claim 16, wherein the exchange of heat of therefrigerant mixture of the first refrigerant mixture cyclecountercurrent to itself is implemented in at least one straight tubeheat exchanger.
 18. The method according to claim 17, wherein the heatexchanger is a plate heat exchanger or a coil-type heat exchanger. 19.The method according to claim 11, wherein a cooling of the refrigerantmixture of the second refrigerant mixture cycle takes placecountercurrent to refrigerant mixture partial streams of the firstrefrigerant mixture cycle in a separate heat exchanger.
 20. The methodaccording to claim 19, wherein the separate heat exchanger is a plateheat exchanger or a coil-type heat exchanger.
 21. The method accordingto claim 12, wherein a cooling of the refrigerant mixture of the thirdrefrigerant mixture cycle takes place countercurrent to refrigerantmixture partial streams of the first refrigerant mixture cycle in aseparate heat exchanger.
 22. The method according to claim 21, whereinthe separate heat exchanger is a plate heat exchanger or a coil-typeheat exchanger.
 23. The method according to claim 12, wherein a coolingof the refrigerant mixture of the second refrigerant mixture cycle takesplace countercurrent to refrigerant mixture partial streams of the firstrefrigerant mixture cycle and a cooling of the refrigerant mixture ofthe third refrigerant mixture cycle takes place countercurrent torefrigerant mixture partial streams of the first refrigerant mixturecycle in dual-stream heat exchangers.
 24. The method according to claim23, wherein the dual-stream heat exchangers are plate heat exchangers.25. A method for liquefying a hydrocarbon-rich stream, comprising thesteps of: liquefaction of the hydrocarbon-rich stream countercurrent toa refrigerant mixture cycle cascade consisting of two refrigerantmixture cycles, wherein a first refrigerant mixture cycle is used forprecooling and a second refrigerant mixture cycle is used forliquefaction and supercooling of the hydrocarbon-rich stream to beliquefied, wherein no additional process steps are involved in a heatexchange between the hydrocarbon-rich stream which is to be precooledand the refrigerant mixture of the first refrigerant mixture cycle. 26.A method for liquefying a hydrocarbon-rich stream, comprising the stepsof: liquefaction of the hydrocarbon-rich stream countercurrent to arefrigerant mixture cycle cascade consisting of at least two refrigerantmixture cycles; wherein a first refrigerant mixture cycle precools thehydrocarbon-rich stream and wherein a second refrigerant mixture cycleliquefies and supercools the hydrocarbon-rich stream; and wherein noadditional process steps are involved in a heat exchange between thehydrocarbon-rich stream and the first refrigerant mixture cycle in theprecooling step.
 27. The method according to claim 26, wherein theliquefaction of the hydrocarbon-rich stream countercurrent to therefrigerant mixture cycle cascade includes a third refrigerant mixturecycle; wherein the second refrigerant mixture cycle liquefies thehydrocarbon-rich stream and wherein the third refrigerant mixture cyclesupercools the liquefied hydrocarbon-rich stream.